The present invention relates to a method of and an apparatus for setting focusing control into operation in a focusing control loop for bringing a focal point of a beam onto a recording surface of an optical disk by driving an objective lens of an optical disk apparatus.
As an apparatus for performing focusing control, an optical reproducing apparatus is known from, for example, Japanese Patent Laid-Open Publication No. 64-89027 in which a beam emitted from a light source such as a semiconductor laser diode is irradiated, through convergence, onto an optical disk rotating at a predetermined number of revolutions such that signals recorded on the optical disk are reproduced. In this known apparatus, focusing control is performed such that a focal point of the beam is at all times located on a recording surface of the optical disk when signals recorded on the recording surface of the optical disk are reproduced, while reflected light from the recording surface is received by a photodetector. A focusing error signal for performing focusing control for bringing the focal point of the beam onto the recording surface at all times is also detected from reflected light from the recording surface. In order to perform focusing control, this focusing error signal is fed back to a focusing actuator for displacing the beam substantially perpendicularly to the recording surface.
Meanwhile, in general focusing error detecting methods of detecting a distance between the focal point of the beam and the recording surface, its detection range is about 10 plus several .mu.m. Therefore, in the case where focusing control is performed, the focusing actuator should be driven preliminarily so as to bring the focal point of the beam within the detection range of the focusing error.
Furthermore, several conditions are required to be satisfied for securing that focusing control is in operation. The most important condition is a relative velocity between the focal point and the recording surface at the time when the focusing control loop is closed. This relative velocity is restricted by the detection range of the focusing error, a gain of the focusing control loop, etc. When the relative velocity exceeds a certain value, the relative velocity falls out of the detection range of the focusing error due to transient response, so that it becomes impossible to set focusing control into operation. Therefore, the focusing actuator is required to be driven gradually such that the relative velocity at the time of closing of the focusing control loop does not exceed a limit velocity.
However, in the known apparatus, the focusing actuator is driven by open loop control. Thus, when a drive signal is lower than a certain level, the focusing actuator is not driven by influences such as wear, etc. Meanwhile, when the drive signal exceeds the certain level, the drive of the focusing actuator is started abruptly. Hence, a velocity at the time when the focal point coincides with the recording surface exceeds the set relative velocity and thus, it frequently becomes impossible to set the focusing control into operation.